1. Field
The present invention relates to an optical film, a surface light source device, and a liquid crystal display device.
2. Related Background Art
As an optical film having a function to enhance luminance when used in a liquid crystal display panel, a reflective polarizing optical film has been proposed (see Japanese Translated International Application Laid-Open No. 11-509331). This optical film is manufactured by alternately stacking 800 or more in total of an optical material layer composed of polyethylene naphthalate and an optical material layer composed of glycol-modified dimethyl cyclohexane terephthalate and stretching them.
The optical film has polarization separating and wavelength selectivity functions. In the visible light wavelength range of 400 to 700 nm, the optical film transmits therethrough 89.7% of light in a specific polarization direction and reflects a light polarized perpendicular to the former polarization direction. The variability of the transmittance of the optical film in the wavelength range of 400 to 700 nm is 1.05% and thus the optical film can uniformly transmit the visible light.
The optical film is disposed closer to an observer than is a light source of the liquid crystal display panel. In the light emitted from the light source, the optical film transmits therethrough a light component polarized parallel to the transmission axis of a polarizing plate on the backlight side of the liquid crystal display panel and reflects a light component polarized perpendicular thereto back to the backlight side. The light returned to the backlight side is emitted again to the observer with a partly changed polarization direction by a reflecting plate disposed on the opposite side of the light source from the observer. Hence, the light reflected with a partly changed polarization direction by the reflective part is reused (recycled), so that the polarized light component parallel to the polarization direction of the optical film is transmitted through the optical film and emitted from the screen of the transmissive liquid crystal display panel. Thus, the optical film exhibits a function to enhance the luminance of the liquid crystal display panel.